Clay detection and quantification using downhole low frequency electromagnetic measurements

ABSTRACT

Methods and systems are provided for clay detection, clay typing, and clay volume quantification using downhole electromagnetic measurements conducted by a downhole logging tool on a formation at a low frequency less than 5000 Hz. The downhole electromagnetic measurements are used to determine permittivity data that characterizes permittivity of the formation at the low frequency less than 5000 Hz. The downhole low frequency electromagnetic measurements are nondestructive, and the results indicate it is with high sensitivity to the existence of clays.

FIELD

The present disclosure relates to methods and systems that can detect clay in a rock sample and that can also determine data characterizing clay type and volume fraction of clay in the rock sample.

BACKGROUND

Geological knowledge of formation rock is important for resources exploration, field development and production planning. To assess reservoir quality and the amount of hydrocarbons in-place, characterization of the mineralogy of the formation rock is needed in order to accurately calculate reservoir porosity and hydrocarbon saturation from logs. This is particularly true in the event that clay is present in the formation rock. Each hydrocarbon reservoir can contain different types of clays and different amounts of such clay types. Each clay type has its own different characteristics, which can be translated to what is called cation exchange capacity (CEC) in common petrophysical applications.

Clays are hydrous aluminum silicate minerals that are platy in structure and can form by the alteration of silicate minerals like feldspar and amphibole. Clays are commonly grouped into a number of clay types, including but not limited smectite, kaolinite, chlorite, illite. Some clays have a tendency to swell when exposed to water, creating a potential drilling hazard when clay-bearing rock formations are exposed to water-base fluids during drilling, possibly reducing the permeability of a good reservoir rock. Some clays are used in drilling fluids to form an impermeable mudcake to isolate a formation from the invasion of drilling fluid. The structural differences amongst the clay types (smectite, kaolinite, chlorite, illite) can determine the surface area exposed to reservoir fluids or stimulating fluids. Clays can be found in pore spaces, as part of the matrix or as grain-cementing material. Authigenic clays, which grow in the pores from minerals in the connate water, can be pore-filling or pore-lining. These clays have considerable surface area exposed in the pore and can be reactive, while detrital clays that are part of the matrix are usually less reactive. Additionally, clays can be cementing, or grain-binding, materials that react with water or acid to disaggregate the formation if they are not protected by quartz overgrowths. The most common clays that create clay problems in hydrocarbon reservoirs are kaolinite, smectite, illite and chlorite.

Clays in formation rock can significantly impact estimation of hydrocarbon reserves and the methods and costs for producing the stored hydrocarbons from formation rock. In evaluating unconventional formation rock such as shale that contains clays, determining clay type and composition is a significant challenge considering the chemical complexity and heterogeneous nature of the unconventional formation rock.

In the downhole environment, gamma ray logs have been traditionally used to estimate formation shale/clay volume based on correlation related to inherent elements such as potassium, uranium and thorium.

Combining density, neutron and spectral natural gamma ray logs, one can determine clay density and neutron response to 100% clay at every depth and also determine clay volume independent of the clay types. Combining such knowledge allows the calculation of CEC and hydrogen index (HI), and from the CEC vs. HI crossplot, clay typing is possible. Nonetheless, such measurements do not have enough sensitivity to allow for the determination of complex mineralogy.

In a laboratory setting, there are multiple techniques for elemental and mineralogical analysis for rock samples, most important are X-ray fluorescence (XRF), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), and Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) in addition to qualitative thin section analysis under polarized transmitted light. In all cases, it is necessary to prepare the required samples to certain standards.

XRF is an electroscopic technique that employs a secondary or fluorescent emission generated by exciting a sample with a source of X-radiation. The energy absorbed by the atoms cause the production of secondary x-ray and fluorescence, emitted by the sample. The intensity of these secondary x-rays is proportional to the concentration of each element in the sample. XRF allows fast identification of elements heavier than Lithium (Z=3) in theory. But in practice, it is often difficult to quantify elements lighter than Sodium (Z=11). Currently, portable fluorescence devices allow real-time measurements, which translate to fast decision making on the field. They offer the possibility to analyze samples without the need of any previous preparation. Despite the advances in portable devices, the accuracy that can be expected from its results is still lower than the one that can be obtained by means of laboratory analytic techniques although it can be enhanced using reference samples. It is also noted that XRF measures elements, converting elements to minerals is an inversion process which may require calibrations and boundary conditions to ensure converging solutions.

In XRD, when an x-ray diffraction beam strikes the surface, the matter absorbs the radiation to a greater or lesser extent, depending on the different mechanisms of interaction that are registered, either fluorescent type or disperse radiations. The dispersive one constituted by the fraction of the incident energy that is emitted again without changing its wavelength. X-ray diffraction is a particular case of this type of radiation. The constant distances of each crystalline structure originate a characteristic distribution of maximums (peaks), which allows identifying the crystals qualitatively; the intensity of these peaks is proportional to the number of planes that diffract the incident beams, what happens just with certain angles of incidence. In such a case, a semi-quantitative concentration of a specific structure can be obtained by analyzing the area under the curve. Each peak is assigned to an intensity value and, with this information, the crystal or crystals to which the diffraction pattern investigated belongs are identified. Peaks of some minerals overlap and therefore depending on their concentration, minerals with little concentration can be completely mischaracterized with others. This can create large quantification errors in sample mineral analysis which can be above 20% depending on the characterized mineral. In addition, for low angle diffraction lines (peaks) such as clays, the signal to noise ratio is usually poor, and due to the amount of disorders that can occur in clays structures, quantifying clays accurately is often difficult. It is also noted that XRD works only for crystalline, not for amorphous materials.

FTIR have been extensively used for mineral identification. Generally, sample preparation is extensive for FTIR. Specifically, potassium bromide (KBr) is mixed with specific mesh size sample powder and then pressed into a pellet and heated at 120° C. to remove water. Data is usually recorded in the range of 4000 to 400 cm⁻¹ using a spectral resolution of 4 cm⁻¹. FTIR can provide chemical and mineral information on complex samples such as shales without the need for separating clays. Although FTIR is a powerful technique for mineral characterization, it does have some limitations with respect to clay quantification.

DRIFTS has recently been introduced as a commercial service for material mineralogy characterization. It is a fast and efficient technique for quantifying mineralogy from OBM-free core and cuttings samples. DRIFTS can be used in a laboratory or at a wellsite. It is not a downhole technique. It measures the vibrational absorbance due to chemical bonds and is capable of resolving illite, smectite, kaolinite, and chlorite. Samples are usually scanned as bulk powders without dilution in reflectance mode and spectra are collected over the mid-IR range from 375 to 4000 cm⁻¹ with 4 cm⁻¹ resolution.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In accordance with the subject disclosure, methods and systems are provided for clay detection, clay typing, and clay volume quantification using downhole electromagnetic measurements on a subsurface formation conducted by a logging tool. The downhole electromagnetic measurements are conducted at a low frequency less than 5000 Hz and used to determine and store permittivity data that characterizes permittivity of the formation at the low frequency less than 5000 Hz. The downhole low frequency electromagnetic measurements are nondestructive, and the results indicate the methods and systems are highly sensitive to the existence of clays.

In embodiments, the methods and systems can employ a computational model that relates a parameter extracted from measurement of permittivity of a formation at a low frequency less than 5000 Hz to data that characterizes at least one clay type and corresponding clay volume fraction. Downhole electromagnetic measurements can be conducted on a formation of interest at a low frequency less than 5000 Hz, and the results of such downhole electromagnetic measurements can be used to determine and store permittivity data that characterizes permittivity of the formation of interest at the low frequency less than 5000 Hz. A parameter can be extracted from the permittivity data. The extracted parameter can be used as input to the computational model, wherein the computational model outputs data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest. The data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as provided by the computational model can be stored or output for use, such as for use in evaluating the formation of interest.

In embodiments, the computational model can be derived by measuring permittivity of formations of different known clay types and different clay volume fractions at the low frequency less than 5000 Hz (such as multiple low frequencies less than 5000 Hz) and correlating a parameter extracted from the resultant permittivity to data that characterizes at least one clay type and corresponding clay volume fraction.

In embodiments, the computational model can relate a parameter extracted from measurement of permittivity of a formation at multiple low frequencies less than 5000 Hz to data that characterizes at least one clay type and corresponding clay volume fraction. The permittivity data of the formation of interest as well as the parameter extracted from the permittivity data can be derived from the electromagnetic measurements of permittivity of the formation of interest at the multiple low frequencies less than 5000 Hz.

In embodiments, the multiple low frequencies less than 5000 Hz can comprise at least three frequencies less than or equal to 100 Hz (and possibly a set of at least three frequencies between 100 Hz and 1 Hz).

In embodiments, the parameter of the computational model as well as the parameter extracted from the measurement of permittivity of the formation of interest can be selected from the group consisting of a frequency-specific slope, a frequency-specific permittivity, a critical frequency where the measurement of permittivity of the formation of interest diverges from permittivity of a formation that does not have clay, and combinations thereof.

In embodiments, the at least one clay type can be selected from the group consisting of kaolinite, smectite, illite, chlorite, and combinations thereof.

In embodiments, the porous media sample can be a formation rock sample. In this case, the data that characterizes at least one clay type and corresponding clay volume fraction as output from the computational model can be used to calculate a value of cation exchange capacity (CEC) for the formation rock sample. The data that characterizes at least one clay type and corresponding clay volume fraction as output from the computational model can also be used for evaluation of a hydrocarbon reservoir corresponding to the formation rock sample.

In embodiment, the extracting of the parameter can be performed by a processor. The computational model can also be embodied by a processor. The operations of the method or system or parts thereof can also be controlled by a processor.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

FIG. 1 is a schematic diagram illustrating an electrical double layer model of interfacial polarization of clay particles;

FIG. 2 depicts plots of relative permittivity ε_(r) computed as a function of frequency (in Hz) for a shaly sandstone formation rock under different saturation conditions (including 10% water saturation/90% oil saturation, 40% water saturation/60% oil saturation, 60% water saturation/40% oil saturation, 80% water saturation/20% oil saturation, and 100% water saturation/0% oil saturation);

FIG. 3 depicts plots of relative permittivity ε_(r) computed as a function of frequency (in Hz) for quartz mixed with smectite of different volume fractions (including 0.1%, 0.5%, 1%, 2%, 5%, 10%, 30% and 50%). For comparison, the relative permittivity ε_(r) computed as a function of frequency for clean sandstone (pure quartz) is also plotted. All the curves have a water saturation level of 20% and an oil saturation level of 80%;

FIG. 4 depicts a correlation curve that represents a relationship between relative permittivity at 1 Hz (ε_(r), 1 Hz) and a volume fraction of smectite clay (V_(smectite)) using data extracted from the measurements of FIG. 3;

FIG. 5 depicts a plot of relative permittivity ε_(r) computed as a function of frequency (in Hz) for sandstone samples mixed with different clay types (kaolinite, illite, chlorite and smectite) of the same 20% volume fraction with a total porosity of 30%. For comparison, the relative permittivity ε_(r) computed as a function of frequency for a clean sandstone sample (no clay) is also plotted. All the curves have a water saturation level of 20% and an oil saturation level of 80%;

FIG. 6A is a block diagram of a well logging system that can incorporate aspects of the subject disclosure;

FIG. 6B is a schematic illustration of a logging tool that can be part of the well logging system of FIG. 6A;

FIG. 7 is a flowchart illustrating a methodology for clay detection, clay typing, and clay volume quantification using a logging tool (such as the logging tool of FIGS. 6A and 6B) for downhole electromagnetic measurements performed on a formation of interest and dispersed at low frequencies (below 5000 Hz). The downhole electromagnetic measurements are used to determine permittivity of the formation of interest at the low frequencies (below 5000 Hz);

FIG. 8 is block diagram of a computer processing system, which can be used to embody parts of the methodology for clay detection, clay typing, and clay volume quantification as described herein;

FIG. 9 is a schematic diagram illustrating the combination of mineralogy data obtained from low-frequency downhole electromagnetic measurements with other minerology log data (such as minerology data obtained from conventional nuclear-based measurements) in order to characterize and evaluate lateral reservoir geological heterogeneity in a vertical wellbore environment.

DETAILED DESCRIPTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.

Interfacial polarization can be observed in formation rock containing clays (such as shaly sands) and in other porous media containing clays. When the surface of a nonconductive mineral, such as clay minerals and silica grains, are exposed to electrolytes, it acquires charges due to ionic adsorption, protonation/deprotonation of the hydroxyl groups, and dissociation of other potentially active surface groups. In the presence of an external electromagnetic (EM) field, these surface charges form electric dipoles and cause interfacial polarization (IP) effects. The strength of the IP effects is regulated by permittivity of the formation rock or other porous media.

The present disclosure provides a framework or methodology for clay detection, clay typing, and clay volume quantification using a downhole logging tool for downhole electromagnetic measurements on a formation of interest that are dispersed at low frequencies (below 5000 Hz). The low frequency downhole electromagnetic measurements are used to determine permittivity of the formation of interest at the low frequencies (below 5000 Hz). The low frequency downhole electromagnetic are nondestructive, and the results indicate the methodology is with high sensitivity to the existence of clays compared to other conventional specialized mineralogical and elemental clay quantification methods such as x-ray diffraction, x-ray fluorescence, and Fourier-transform infrared spectroscopy.

In embodiments, clay minerals can be lumped together on the basis of molecular structure and composition into four commonly encountered and representative clay types: kaolinite, illite, chlorite and smectite. Although each one of these clay types impacts formation conductivity differently, the fundamental mechanism is similar. When the surface of a clay mineral grain is exposed to electrolytes, it acquires charges due to ionic adsorption, protonation/deprotonation of the hydroxyl groups, and dissociation of other potentially active surface groups.

Under an external electromagnetic (EM) field, both electrical conduction (due to charge carries) and interfacial polarization (due to surface charges) influences the measured EM fields. Electrical conduction describes the movement of the charge carries under the influence of the external EM field. This is a well understood phenomena and can be described by Ohm's law.

The polarization of clay particles is mostly due to charge accumulation and movements at host-inclusion interfaces. The most common theory to describe the interfacial polarization is the electrical double layer shown in FIG. 1. At the surface of the clay particles, both Stern and diffuse layers are formed due to charge absorptions and movements. In the presence of an applied external electromagnetic field, the double layer develops a counter ion cloud and diffused-charge distribution around host-inclusion interfaces. Dynamics of accumulation/depletion of charge concentrations around host-inclusion interfaces influence the magnitude and phase of the EM response of the formation rock containing clay minerals or other porous media containing clay minerals. The electrical conductivity of the porous media can be described by a complex conductivity σ given by:

σ=σ^(R) +iωε ₀ε_(r), and   Eqn. (1a)

σ=σ^(R) +iωε _(eff)   Eqn. (1b)

where σ^(R) is the in-phase component and ωε₀ε_(r) is the quadrature (out-phase) component of a complex conductivity σ, respectively;

-   -   ω is frequency,     -   ε₀ is the permittivity of vacuum (e.g., 8.854×10⁻¹² F m¹),     -   ε_(r) is the relative permittivity, and     -   ε_(eff) is the effective permittivity given by the product ε₀         ε_(r).

For a porous media containing clay minerals, the relative permittivity ε_(r) and the effective permittivity ε_(eff) depend on clay type and volume of each clay type. In addition, the relative permittivity ε_(r) and the effective permittivity ε_(eff) can be calculated based on effective media theory. In embodiments, the EM response signal of the porous media can be detected by one or more receiver antennae (e.g., receiver coil(s)). Phased-lock detection and amplification of the EM response signal can determine the amplitude and phase of the voltage levels detected by the one or more receiver antennae. By recording and processing the amplitude and phase of such voltage levels, a measurement of the complex conductivity σ of the porous media can be obtained. The relative permittivity ε_(r) and/or the effective permittivity ε_(eff) of the porous media can be extracted from the quadrature component of the complex conductivity σ according to eqns. (1a) and (1b) as set forth above.

FIG. 2 shows relative permittivity ε_(r) (no unit) computed as a function of frequency (in Hz) for a shaly sandstone formation rock under different saturation conditions (including 10% water saturation/90% oil saturation, 40% water saturation/60% oil saturation, 60% water saturation/40% oil saturation, 80% water saturation/20% oil saturation, and 100% water saturation). The shaly sandstone formation rock has 90% quartz and 10% smectite with a total porosity of 30%. Very strong dispersion effects are observed below 1000 Hz. The largest permittivity value can reach 2×10⁴ (F m¹) at 1 Hz. For frequencies larger than 1000 Hz, the permittivity has much smaller values and is non-dispersive. Note that while parts of the curves show dependence on saturations, the dispersion section of the curves, for example in frequencies less than 100 Hz, is very insensitive to saturation. This implies that the measurement of relative permittivity ε_(r) of formation rock as a function of frequency, at low frequencies less than 100 Hz, is a sensitive indicator for clay detection and quantification of clay volumes irrespective of the saturation conditions of the formation rock.

FIG. 3 shows relative permittivity ε_(r) (no unit) computed as a function of frequency (in Hz) for quartz mixed with smectite of different volume fractions (including 0.1%, 0.5%, 1%, 2%, 5%, 10%, 30% and 50%). For comparison, the relative permittivity ε_(r) (no unit) computed as a function of frequency for clean sandstone (pure quartz) is also plotted. All the curves have a water saturation level of 20% and an oil saturation level of 80%. It is observed that while the clean sandstone has no dispersion effect, the shaly sandstone shows strong dispersion effects. Specifically, the relative permittivities of the formation rock samples at a low frequency such as 1 Hz strongly depends on the respective volume fractions of the smectite clay component in the formation rock samples. Even with 1% or less clay content, the dispersion curves show a huge difference from the clean sandstone, indicating the measurement of relative permittivity of the formation rock sample at a low frequency (such as, for example, 1 Hz) is a very sensitive technique to detect small amount of clays for characterization of the clay components of the formation rock sample. In addition, the descending values of the relative permittivities of the formation rock samples at the low frequency (such as, for example, 1 Hz) are easily differentiable for different volume fractions of the smectite clay component in the formation rock samples. The sensitivity to differentiate clean and slightly shaly rock appears to be an improvement over downhole nuclear elemental spectroscopy measurements as described herein.

In order to quantify clay volume, a computational model can be built that relates measurements of relative permittivity (or effective permittivity) at one or more low frequencies less than 5000 Hz to clay volume fraction. In embodiments, the computational model can relate measurements of relative permittivity (or effective permittivity) at multiple (at least three) low frequencies (e.g. 1 Hz, 10 Hz and 100 Hz) to clay volume fraction. For example, measurements of relative permittivity of a formation rock sample at 1 Hz can be related to clay volume fraction as shown in the correlation curve of FIG. 4, which shows a relationship between relative permittivity at 1 Hz (ε_(r), 1 Hz) and a volume fraction of smectite clay (v_(smectite)) using data extracted from the measurements of FIG. 3. This relationship demonstrates the sensitivity of low frequency permittivity on smectite clay volume and can be represented by:

$\begin{matrix} {ɛ_{r},{{1{Hz}} = {{1856.7*V_{smectite}} + 7.0}},{or}} & {{Eqn}.\mspace{14mu}\left( {2a} \right)} \\ {v_{smectite} = {\frac{ɛ_{r,{1{Hz}}} - ɛ_{r,{{clean}\mspace{14mu}{rock}}}}{Slope} = \frac{ɛ_{r,{1{Hz}}} - 7.0}{1856.7}}} & {{Eqn}.\mspace{14mu}\left( {2b} \right)} \end{matrix}$

Note that the relationship of FIG. 4 and Eqns. (2a) and (2b) is an example, and thus other correlations and computational models can be used depending on the clay type, clay volume and a mixture of different clay types and clay volumes. Laboratory measurements of relative permittivity (or effective permittivity) at the low frequency(ies) on reservoir rock samples of known or measured clay volumes can be used to determine the relationship between measured permittivity at one or more low frequencies and clay volume fractions individually or in mixtures.

FIG. 5 shows relative permittivity ε_(r) (no unit) computed as a function of frequency (in Hz) for sandstone samples mixed with different clay types (kaolinite, illite, chlorite and smectite) of the same 20% volume fraction with a total porosity of 30%. For comparison, the relative permittivity ε_(r) (no unit) computed as a function of frequency for a clean sandstone sample (no clay) is also plotted. All the curves have a water saturation level of 20% and an oil saturation level of 80%. It is clearly observed that smectite and kaolinite have different relative permittivities at a low frequency such as 1 Hz, which are easily distinguishable from one another. More specifically, the sandstone sample mixed with kaolinite at 20% volume fraction has a relative permittivity on the order of 40, while the sandstone sample mixed with smectite at 20% volume fraction has a relative permittivity ε_(r) on the order of 2.5×10⁴. Furthermore, it is clearly observed that smectite and kaolinite have different critical frequencies where the measured relative permittivity deviates significantly from that of clean sandstone. More specifically, the sandstone sample mixed with kaolinite at 20% volume fraction has a critical frequency at or near 10 Hz, while the sandstone sample mixed with smectite at 20% volume fraction has a critical frequency near 900 Hz. Note that these two parameters of the measured permittivity at a low frequency cannot be used to distinguish the illite and chlorite clay types because these two clay types have almost the same electric properties. In practical applications, permittivities at multiple low frequencies (e.g. 1 Hz, 10 Hz and 100 Hz) can be measured on core samples of the known different clay types (including the kaolinite, illite, chlorite, smectite and combinations of such clay types) and of known different clay type volume fractions, and such measurements can be processed in order to build a computational model (such as a correlation model) that relates parameters extracted from low frequency permittivity measurements into one or more clay types and corresponding clay type volume fractions.

The following conclusions can be based on FIGS. 2, 3, 4 and 5.

First, measurements of relative permittivity ε_(r) (or effective permittivity ε_(eff)) as a function of frequency at low frequencies have almost no dependence on water saturation of shaly sandstone formation rock. This makes such measurements an ideal clay detector for all saturation conditions, pay zones or otherwise.

Second, the measurements of relative permittivity ε_(r) (or effective permittivity ε_(eff)) of the formation rock at low frequencies (such as less than 1000 Hz) strongly depends on clay volume fractions for shaly sandstone formation rock. Even with 1% or less clay content, the relative permittivity ε_(r) (or the effective permittivity ε_(eff)) of the formation rock at the low frequency shows a huge difference from that of clean sandstone. In addition, for formation rocks with different clay volume fractions, the relative permittivities (or effective permittivities) of the formation rocks at low frequency are easily differentiable for the different clay volume fractions.

Third, the measurements of relative permittivity ε_(r) (or effective permittivity ε_(eff)) of formation rock at low frequencies (such as less than 1000 Hz) also strongly depends on clay types for shaly sandstone formation rock. Furthermore, the presence of smectite and kaolinite clay types that commonly occur in a reservoir environment can be detected based on the measurements of relative permittivity ε_(r) (or effective permittivity ε_(eff)) of formation rock at low frequency (such as less than 1000 Hz). For example, the measurement of relative permittivity ε_(r) (or effective permittivity ε_(eff)) of formation rock at a low frequency such as 1 Hz and/or the critical frequency that relative permittivity ε_(r) (or effective permittivity ε_(eff)) of formation rock deviates from that of clean sandstone can be used to detect the presence of smectite and kaolinite clay types. Note that these measurements cannot distinguish between illite and chlorite clay types as these two clays have almost the same electric properties.

Fourth, in order to detect different clays types (including smectite, kaolinite, illite and chlorite and combinations thereof) and to quantify the corresponding volume fraction of the different clay types, relative permittivity ε_(r) (or effective permittivity ε_(eff)) of the formation rock can be measured at multiple low frequencies below 5000 Hz. One or more parameters (such as a frequency-specific slope or frequency-specific measure(s) of permittivity) can be extracted from the multiple low frequency permittivity measurements and used in conjunction with a computational model (such as a correlation model) that relates such parameters to one or more clay types (such as smectite, kaolinite, illite, chlorite, and combinations thereof) and corresponding clay type volume fractions.

During the exploration and production of oil and gas, many well logging techniques can be deployed to log data of a subsurface formation. The data contain information that can be used to locate subsurface hydrocarbon reservoirs and to determine types and quantities of subsurface hydrocarbons. In such logging processes, a tool may be lowered into a borehole traversing a subsurface formation, either after the well has been drilled or during the drilling process. A typical logging tool includes a “sonde”, that emits, for example, acoustic or EM waves to interact with the surrounding formation. The signals produced from such interactions are then detected and measured by one or more sensors on the instrument. By processing the detected signals, a profile or log of the formation properties can be obtained.

Logging techniques known in the art include “wireline” logging, logging-while-drilling (LWD), measurement-while-drilling (MWD), and logging-while-tripping (LWT). Wireline logging involves lowering an instrument into an already-drilled borehole at the end of an electrical cable to obtain measurements as the instrument is moved along the borehole. LWD and MWD involve disposing an instrument in a drilling assembly for use while a borehole is being drilled through earth formations. LWT involves disposing sources or sensors within the drill string to obtain measurements while the string is being withdrawn from the borehole.

Turning now to FIG. 6A, a wireline logging system 1 is shown that includes an electromagnetic (EM) logging tool 10 which is suspended via a cable 12 in a borehole 14 which traverses a formation 15. The cable 12 is wound about a winch 17 or suitable suspension means located at the surface of the earth formation, and may be utilized, if desired, to carry data (information) which is sent by the tool 10 to a processor 20.

As is well known in the art, the gathered data may be preprocessed downhole by a processor (not shown) associated with the tool 10 and may be sent via the cable 12, or via wireless mechanisms (e.g., mud pulsing) to the surface-located processor 20 for additional processing. The processor 20 may be located in the vicinity of the formation 15 or at another site as desired. Alternatively, raw data may be sent to the processor 20. As has been previously established, the mudcake on the borehole wall may be relatively conductive in the case where water-base mud is used in the borehole, or may be relatively resistive in the case where oil-base mud is used in the borehole. It is desirable that the tool 10 can be configured for use in both situations. In other embodiments, the logging tool 10 can be an LWD logging tool or MWD logging or LWT logging tool.

EM logging tools are widely used for formation logging applications. EM logging tools include antennas that are operable as transmitters and/or receivers. The antennas are typically solenoid coils. During operation, a coil may function as a transmitter antenna when it is energized with an alternating current or an oscillating electrical signal. The transmitter antenna emits EM waves through the borehole mud and into the surrounding earth formation. The same coil or another coil may function as a receiver antenna that collects EM signals carrying information about the interactions between the EM waves and the mud/formation.

In conventional EM logging tools, the transmitter and receiver antennas are mounted with their axes aligned with the longitudinal axis of the instrument. Thus, these tools are implemented with antennas having longitudinal magnetic dipoles (LMD). When an LMD antenna is placed in a borehole and energized to transmit EM energy, the induced electric currents flow around the antenna in the borehole and in the surrounding earth formations, and no net current flows up or down the borehole.

More recent EM well logging tools have tilted or transverse coils, i.e., the coil's axis is not parallel with the longitudinal axis of the support. Consequently, the antenna has a transverse or tilted magnetic dipole (TMD). The TMD configuration permits a tool to have a three-dimensional evaluation capability, such as information about resistivity anisotropy or locations and orientations of dips and faults. In addition, directional sensitivity of the data can be used for directional drilling. Under certain conditions, a TMD-antenna may cause a net current to flow up or down the borehole. Some TMD-antennas are configured with multiple coils. For example, a particular TMD-antenna design includes a set of three coils, and such an antenna is known as a triaxial antenna.

In wireline applications, the antennas are typically enclosed in a housing made of tough non-conductive materials such as a laminated fiberglass material. In LWD applications, the antennas are generally encased into a metallic support so that it can withstand the hostile environment and conditions encountered during drilling. Alternatively, logging instruments may be made of composite materials, thus, providing a non-conductive structure for mounting the antennas.

Induction logging is a well-known form of EM logging. In this type of logging, induction tools are used to produce a conductivity or resistivity profile of earth formations surrounding a borehole.

A conventional induction logging tool or “sonde” may include a transmitter antenna and a receiver antenna. Note that the designation of a transmitter and a receiver is for clarity of illustration. One skilled in the art would appreciate that a transmitter may be used as a receiver and a receiver may also be used as a transmitter depending on the application. Each antenna may include one or more coils, and may be mounted on the same support member or on different support members, i.e., the transmitter antenna and the receiver antenna may be on different tool sections. The antennas are axially spaced from each other in the longitudinal direction of the tool.

In use, the transmitter antenna is energized with an alternating current. This generates an EM field that induces eddy currents in the earth formation surrounding the borehole. The intensity of the eddy currents is proportional to the conductivity of the formation. The EM field generated by the eddy currents, in turn, induces an electromotive force in one or more receiving coils. Phase-locked detection, amplification, and digitization of this electromotive force signal determines the amplitude and the phase of the voltage on the receiver coil. By recording and processing the receiver voltages, a measurement of complex conductivity of the earth formation can be obtained. The relative permittivity and/or the effective permittivity of the earth formation can be extracted from the quadrature component of the complex conductivity according to eqns. (1a) and (1b) as set forth above.

FIG. 6B illustrates an induction logging tool 10 that includes a transmitter coil 110, a receiver coil 112 for probing a shallow depth into the formation, a receiver coil 115 for probing a medium depth into the formation, and a receiver coil 118 for probing deeper into the formation. Buckling coils or trim coils 111, 114, 113, 116, 117 and 119 are provided to eliminate or reduce direct coupling between the transmitter coil 110 and the receiver coils 112, 115 and 118. In addition, the logging tool 10 can also include one or more electrodes (one shown as 120 in FIG. 6B), such as those used in conventional conductivity/resistivity tools. Details of the induction logging tool 10 are set forth in U.S. Pat. No. 7,501,829, commonly assigned to assignee of the subject application and herein incorporated by reference in its entirety. The transmitter antenna coil 110 can be energized with an alternating current of desired frequency (in Hz). This generates an EM field that induces eddy currents in the earth formation surrounding the borehole. The intensity of the eddy currents is proportional to the conductivity of the formation. The EM field generated by the eddy currents, in turn, induces an electromotive force in the receiver coils 112, 115, 118. Phase-locked detection, amplification, and digitization of this electromotive force signal determines the amplitude and the phase of the voltage on the respective receiver coils 112, 115, 118. By recording and processing the receiver voltage signal sensed by the receiver coils 112, 115, 118, measurements of complex conductivity of the earth formation can be obtained for three different radial depths (shallow/medium, deep) into the formation. The relative permittivity and/or the effective permittivity of the earth formation at the respective depth locations (shallow, medium, deeper) can be extracted from the quadrature component of the complex conductivity measurements according to eqns. (1a) and (1b) as set forth above.

FIG. 7 depicts a workflow for clay detection, clay typing, and clay volume quantification using a logging tool (such as the logging tool of FIGS. 6A and 6B) for downhole electromagnetic measurements performed on a formation of interest and dispersed at low frequencies (below 5000 Hz). The downhole electromagnetic measurements are used to determine permittivity of the formation of interest at the low frequencies (below 5000 Hz).

In block 701, a computational model is built (or provided) that relates relevant parameters extracted from low frequency permittivity data (which is data derived from low frequency electromagnetic measurements that are used to determine permittivity of a formation) to data characterizing clay type(s) and clay volume fraction(s) of the formation. The computational model can be built by configuring a downhole EM logging tool (such as the logging tools of FIGS. 6A and 6B) to conduct downhole electromagnetic measurements and determine permittivity of formations of known different clay types (including kaolinite, illite, chlorite, smectite and combinations of such clay types) and of known different clay type volume fractions at multiple low frequencies (e.g. 1 Hz, 10 Hz and 100 Hz). Such low frequency downhole electromagnetic measurements can measure complex conductance of the formation. Permittivity data can be determined from the complex conductance of the formation. The permittivity data can represent effective permittivity or relative permittivity of the formation. The permittivity data that results from such low frequency downhole electromagnetic measurements can be processed to extract one or more relevant parameters from the low frequency permittivity data (such as a frequency-specific slope or frequency-specific measure(s) of permittivity or critical frequency) and determine a correlation function or other data structure that describes the relationship between the extracted relevant parameter(s) and one or more clay types and corresponding clay type volume fractions. The correlation function or other data structure can then be integrated as part of the computational model. The computational model can be designed to take as input one or more relevant parameters extracted from low-frequency permittivity data. The correlation function or other data structure of the computational model is used to output one or more clay types and corresponding clay type volume fractions that correspond to the relevant parameter(s) supplied as input to the computational model.

Although current laboratory techniques do not have the capabilities to detect low concentration of clays, calibration curves can be built based on artificial cores where a known amount of clay is synthesized and prepared for the different clay types as individual and in a mixed known amount. Resistivity measurements can be conducted and at different frequencies and a calibration curve/model can be built and used to identify clay and its volume. A similar or close calibration curve or model can be produced using the current available inversion models.

In block 703, a downhole EM logging tool (such as the EM logging tool of FIGS. 6A and 6B) is configured to measure and store conductivity data representing conductivity of a formation of interest at multiple low frequencies less than 5000 Hz (e.g. 1 Hz, 10 Hz and 100 Hz). In this block 702, the downhole EM logging tool can be located in a desired depth location in a borehole that traverses the formation of interest and operated to measure complex conductivity data representing conductivity of the formation of interest at the multiple low frequencies less than 5000 Hz (e.g. 1 Hz, 10 Hz and 100 Hz). For each one of the multiple low frequencies (e.g. 1 Hz, 10 Hz and 100 Hz) of the experiment, the frequency of the applied time varying external electromagnetic field produced by the transmitter antenna of the downhole EM logging tool can be controlled to correspond to the particular low frequency, and the electromagnetic response of the formation of interest can be measured by the downhole EM logging tool for that particular low frequency.

In block 705, the conductivity data measured and stored in block 703 is processed to extract permittivity data representing permittivity of the formation of interest at the multiple low frequencies less than 5000 Hz (e.g. 1 Hz, 10 Hz and 100 Hz). Such permittivity data can represent effective permittivity or relative permittivity of the formation of interest at the multiple low frequencies less than 5000 Hz (e.g. 1 Hz, 10 Hz and 100 Hz). For example, the relative permittivity and/or the effective permittivity of the formation of interest can be extracted from the quadrature component of the complex conductivity of the formation of interest according to Eqn. (1a) and (1b) as set forth above.

In blocks 707, 709, 711 and 713, the permittivity data measured and stored in block 705, which represents permittivities of the formation of interest as a function of frequency, can be analyzed for clay detection and qualitative clay assessment. For example, the permittivity data measured and stored in block 705 can be plotted in block 707 and the plot (or the permittivity data itself) can be analyzed for clay detection in block 709. For example, the plot (or the permittivity data itself) can be compared to measured permittivity values (e.g. curves) for one or more formations that are known to not have clay as well as to measured permittivity values (e.g. curves) for one or more formations of known clay types. If the permittivity data measured and stored in block 705 is dissimilar to the measured permittivity values (e.g. curves) for the one or more formations that are known to not have clay and similar to the measured permittivity values (e.g. curves) for one or more formations of known clay types, the process can determine that the formation does contain clays in block 711 and continue to blocks 713 to 719. On the other hand, when the permittivity data measured and stored in block 705 is similar to measured permittivity values (e.g. curves) for the one or more formations that are known to not have clay and dissimilar to the measured permittivity values (e.g. curves) for the one or more formations of known clay types, the process can determine in block 711 that the formation does not contain clays and continue to block 721.

In block 713, the permittivity data measured and stored in block 705 can be analyzed for qualitative clay assessment. For example, the permittivity data measured and stored in block 705 can be plotted and compared to measured permittivity values (e.g. curves) for one or more formations of known clay types. The known clay type of the formation whose measured permittivity values best match the permittivity data measured and stored in block 705 can be identified as the clay type for the formation of interest.

In block 715, the permittivity data measured and stored in block 705 can be processed to extract relevant parameter(s) of such data for input to the computational model of block 701.

In block 717, the relevant parameter(s) extracted in 715 can be used as input to the computational model, which outputs data characterizing one or more clay types and corresponding clay volume fractions for the formation of interest. The data characterizing one or more clay types and corresponding clay volume fractions for the formation of interest as output by the computational model can be stored in computer memory for subsequent access or possibly output for analysis (for example, plotted as part of a well log).

In block 719, the process can identify the formation of interest as shaly type and possibly use the data that characterizes the one or more clay types and corresponding clay volume fractions as output from the computational model in block 717 to evaluate the formation of interest. For example, the data that characterizes the one or more clay types and corresponding clay volume fractions as output from the computational model in block 717 can be used to calculate a value of cation exchange capacity (CEC) for the formation of interest. The value of CEC represents a quantity of positively charged ions (cations) that the formation of interest can accommodate on its negative charged surface. It is typically expressed as milliequivalents per 100 grams. The CEC value can be used in formation modeling and simulation of the formation of interest.

For example, if both clay types and clay volume fractions are known for each clay type, the total CEC values can be calculated using a mixing law as follows:

CEC=w _(C)×CEC_(C) +w _(I)×CEC_(I) +w _(K)×CEC_(K) +w _(s)×CEC_(S),   Eqn. (3)

where w_(C), w_(I), w_(K), w_(S) are clay weight fractions for chlorite, illite, kaolinite and smectite, respectively; and CEC_(C), CEC_(I), CEC_(K) and CEC_(S) are CEC values for chlorite, illite, kaolinite and smectite.

The CEC value for each above-mentioned clay mineral is well defined and can be treated as a known parameter. For example, the clay volume can be calculated as follows:

$\begin{matrix} {{V_{cl} = \frac{{\rho_{matrix}\left( {1 - {total}} \right)} \times W_{cl}}{\rho_{cl}}},} & {{Eqn}.\mspace{14mu}(4)} \end{matrix}$

where φ_(total) is the formation total porosity, ρ_(matrix) is matrix density, W_(cl) is weight of clay, and ρ_(cl) is density of clay. In block 721, the process can identify the formation of interest as non-shaly type for evaluation of the formation.

In other examples, for vertical wellbore environments, the mineralogy data that characterizes the one or more clay types and corresponding clay volume fractions as output from the computational model in block 717 can be combined with other minerology log data (such as minerology data obtained from conventional nuclear-based measurements) as part of block 719 in order to characterize and evaluate lateral reservoir geological heterogeneity. This application is shown schematically in FIG. 9. Note that the mineralogy data output from the computational model in block 717 can characterize the mineralogy of the formation rock at a deeper lateral (or radial) offset into the formation relative to the vertical wellbore as compared to the mineralogy data of the other minerology log data, which can characterize the mineralogy of the formation rock at a shallower lateral (or radial) offset into the formation relative to the vertical wellbore.

In yet other examples, for while-drilling applications in horizontal wellbore environments, the minerology data that characterizes the one or more clay types and corresponding clay volume fractions as output from the computational model in block 717 can be used to identify shaly formation rock and control geosteering of the drill bit based thereon as part of block 719 in order to avoid shaly formation rock and maximize reservoir contact and well performance.

FIG. 8 illustrates an example device 2500, with a processor 2502 and memory 2504 that can be configured to implement various embodiments of the logging methodology and logging systems as discussed in this disclosure. Memory 2504 can also host one or more databases and can include one or more forms of volatile data storage media such as random-access memory (RAM), and/or one or more forms of nonvolatile storage media (such as read-only memory (ROM), flash memory, and so forth).

Device 2500 is one example of a computing device or programmable device and is not intended to suggest any limitation as to scope of use or functionality of device 2500 and/or its possible architectures. For example, device 2500 can comprise one or more computing devices, programmable logic controllers (PLCs), etc.

Further, device 2500 should not be interpreted as having any dependency relating to one or a combination of components illustrated in device 2500. For example, device 2500 may include one or more of computers, such as a laptop computer, a desktop computer, a mainframe computer, etc., or any combination or accumulation thereof.

Device 2500 can also include a bus 2508 configured to allow various components and devices, such as processors 2502, memory 2504, and local data storage 2510, among other components, to communicate with each other.

Bus 2508 can include one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. Bus 2508 can also include wired and/or wireless buses.

Local data storage 2510 can include fixed media (e.g., RAM, ROM, a fixed hard drive, etc.) as well as removable media (e.g., a flash memory drive, a removable hard drive, optical disks, magnetic disks, and so forth).

One or more input/output (I/O) device(s) 2512 may also communicate via a user interface (UI) controller 2514, which may connect with I/O device(s) 2512 either directly or through bus 2508.

In one possible implementation, a network interface 2516 may communicate outside of device 2500 via a connected network.

A media drive/interface 2518 can accept removable tangible media 2520, such as flash drives, optical disks, removable hard drives, software products, etc. In one possible implementation, logic, computing instructions, and/or software programs comprising elements of module 2506 may reside on removable media 2520 readable by media drive/interface 2518.

In one possible embodiment, input/output device(s) 2512 can allow a user (such as a human annotator) to enter commands and information to device 2500, and also allow information to be presented to the user and/or other components or devices. Examples of input device(s) 2512 include, for example, sensors, a keyboard, a cursor control device (e.g., a mouse), a microphone, a scanner, and any other input devices known in the art. Examples of output devices include a display device (e.g., a monitor or projector), speakers, a printer, a network card, and so on.

Some of the methods and processes described above, can be performed by a processor. The term “processor” should not be construed to limit the embodiments disclosed herein to any particular device type or system. The processor may include a computer system. The computer system may also include a computer processor (e.g., a microprocessor, microcontroller, digital signal processor, or general-purpose computer) for executing any of the methods and processes described above.

The computer system may further include a memory such as a semiconductor memory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card), or other memory device.

Some of the methods and processes described above, can be implemented as computer program logic for use with the computer processor. The computer program logic may be embodied in various forms, including a source code form or a computer executable form. Source code may include a series of computer program instructions in a variety of programming languages (e.g., an object code, an assembly language, or a high-level language such as C, C++, or JAVA). Such computer instructions can be stored in a non-transitory computer readable medium (e.g., memory) and executed by the computer processor. The computer instructions may be distributed in any form as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over a communication system (e.g., the Internet or World Wide Web).

Alternatively or additionally, the processor may include discrete electronic components coupled to a printed circuit board, integrated circuitry (e.g., Application Specific Integrated Circuits (ASIC)), and/or programmable logic devices (e.g., a Field Programmable Gate Arrays (FPGA)). Any of the methods and processes described above can be implemented using such logic devices.

Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the examples without materially departing from this subject disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. 

What is claimed is:
 1. A method for characterizing clay content of a subsurface formation of interest, the method comprising: i) configuring and operating a downhole logging tool to conduct a downhole electromagnetic measurement on the formation of interest at a low frequency less than 5000 Hz, wherein the downhole electromagnetic measurement is used to determine and store permittivity data that characterizes permittivity of the formation of interest at the low frequency less than 5000 Hz; ii) extracting a parameter from the permittivity data that characterizes permittivity of the formation of interest at the low frequency less than 5000 Hz; and iii) using the parameter extracted from the permittivity data in ii) as input to a computational model, wherein the computational model relates the parameter extracted from the permittivity data in ii) to data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest.
 2. A method according to claim 1, further comprising: storing or outputting the data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as provided by the computational model in iii).
 3. A method according to claim 1, wherein: the computational model of iii) is derived by measuring permittivity of formations of different known clay types and different clay volume fractions at the low frequency less than 5000 Hz and correlating a parameter extracted from the resultant permittivity to data that characterizes at least one clay type and corresponding clay volume fraction.
 4. A method according to claim 1, wherein: the computational model of iii) is derived by measuring permittivity of formations of different known clay types and different clay volume fractions at multiple low frequencies less than 5000 Hz and correlating a parameter extracted from the resultant permittivity to data that characterizes at least one clay type and corresponding clay volume fraction.
 5. A method according to claim 1, wherein: the computational model of iii) relates a parameter extracted from measurement of permittivity of a formation at multiple low frequencies less than 5000 Hz to data that characterizes at least one clay type and corresponding clay volume fraction; the permittivity data determined and stored in i) as well as the parameter extracted from the permittivity data in ii) are derived from electromagnetic measurements conducted by the downhole logging tool on the formation of interest at the multiple low frequencies less than 5000 Hz.
 6. A method according to claim 5, wherein: the multiple low frequencies less than 5000 Hz comprises at least three frequencies less than or equal to 100 Hz.
 7. A method according to claim 5, wherein: the multiple low frequencies less than 5000 Hz comprises a set of at least three frequencies between 100 Hz and 1 Hz.
 8. A method according to claim 1, wherein: the parameter extracted from the permittivity data in ii) and input to the computational model of iii) is selected from the group consisting of: a frequency-specific slope, a frequency-specific permittivity, a critical frequency where permittivity of the formation of interest diverges from permittivity of a formation that does not have clay, and combinations thereof.
 9. A method according to claim 1, wherein: the permittivity data of i) represents relative permittivity or effective permittivity of the formation of interest.
 10. A method according to claim 1, wherein: the permittivity data of i) is derived from a quadrature component of a measurement of complex conductivity of the formation of interest.
 11. A method according to claim 1, wherein: the at least one clay type is selected from the group consisting of: kaolinite, smectite, illite, chlorite, and combinations thereof.
 12. A method according to claim 1, further comprising: using the data that characterizes at least one clay type and corresponding clay volume fraction for the formation as output from the computational model in iii) to calculate a value of cation exchange capacity (CEC) for the formation.
 13. A method according to claim 1, further comprising: using the data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as output from the computational model in iii) for evaluation of the formation of interest.
 14. A method according to claim 1, further comprising: combining the data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as output from the computational model in iii) with other log data (such as conventional nuclear based mineralogy log data) for characterizing and evaluating near wellbore reservoir geological heterogeneity in a vertical wellbore.
 15. A method according to claim 1, further comprising: using the data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as output from the computational model in iii) to identify shaly formation rock and control geosteering of the drill bit while drilling a horizontal wellbore based thereon in order to avoid shaly formation rock.
 16. A method according to claim 1, wherein: the extracting of the parameter in ii) is performed by a processor.
 17. A method according to claim 1, wherein: the computational model of iii) is embodied by a processor.
 18. A system for characterizing clay content of a subsurface formation of interest, the system comprising: a downhole logging tool that is configured to conduct an electromagnetic measurement on the formation of interest at a low frequency less than 5000 Hz, wherein the electromagnetic measurement measures and stores conductivity data that characterizes complex conductivity of the formation of interest at the low frequency less than 5000 Hz; and at least one processor that, when executing program instructions stored in memory, is configured to: i) obtain the conductivity data that characterizes complex conductivity of the formation of interest at the low frequency less than 5000 Hz; ii) process the conductivity data of i) to determine permittivity data that characterizes permittivity of the formation of interest at the low frequency less than 5000 Hz; iii) extract a parameter from the permittivity data of ii); and iv) use the parameter extracted from the permittivity data in iii) as input to a computational model, wherein the computational model relates the parameter extracted from the permittivity data in ii) to data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest.
 19. A system according to claim 18, wherein: the at least one processor is further configured to store or output the data that characterizes at least one clay type and corresponding clay volume fraction for the formation of interest as provided by the computational model in iv).
 20. A system according to claim 18, wherein: the permittivity data determined in ii) is derived from a quadrature component of the complex conductivity of the formation of interest.
 21. A system according to claim 18, wherein: the computational model of iv) is derived by measuring permittivity of formations of different known clay types and different clay volume fractions at the low frequency less than 5000 Hz and correlating a parameter extracted from the resultant permittivity to data that characterizes at least one clay type and corresponding clay volume fraction.
 22. A system according to claim 18, wherein: the computational model of iv) is derived by measuring permittivity of formations of different known clay types and different clay volume fractions at multiple low frequencies less than 5000 Hz and correlating a parameter extracted from the resultant permittivity to data that characterizes at least one clay type and corresponding clay volume fraction.
 23. A system according to claim 18, wherein: the computational model of iv) relates a parameter extracted from measurement of permittivity of a formation at multiple low frequencies less than 5000 Hz to data that characterizes at least one clay type and corresponding clay volume fraction; the permittivity data determined in ii) as well as the parameter extracted from the permittivity data in iii) are derived from electromagnetic measurements conducted by the downhole logging tool on the formation of interest at the multiple low frequencies less than 5000 Hz.
 24. A system according to claim 23, wherein: the multiple low frequencies less than 5000 Hz comprises at least three frequencies less than or equal to 100 Hz.
 25. A system according to claim 23, wherein: the multiple low frequencies less than 5000 Hz comprises a set of at least three frequencies between 100 Hz and 1 Hz.
 26. A system according to claim 18, wherein: the downhole logging tool is selected from the group consisting of: a wireline logging tool, a logging-while-drilling logging tool, a measurement-while-drilling logging tool, and a tripping-while-drilling logging tool. 